In a digital data communication system the transmitted data is generally attenuated and distorted by the medium and the AC coupling networks through which it is transmitted. This results, among other things, in a loss of the low frequency and DC components in the received data.
To combat this problem, receivers typically include a DC (direct current) restorer to restore or regenerate the low frequency and DC components of the transmitted input, and an automatic gain control (AGC) circuit which automatically changes the gain or amplification of the received input to maintain the level of the amplified signal essentially constant despite variations in input signal strength.
DC restorer circuits are generally implemented as either a clamping DC restorer or a DC restorer based on the principle of quantized feedback (QFB). Both clamping and quantized feedback restorer circuits are described in detail in U.S. Pat. No. 5,426,389, the description of said patent being incorporated herein by this reference. A QFB DC restorer circuit generally exhibits superior noise and jitter performance, however such circuits are susceptible to latching-up if the output of the restorer is in the incorrect state at the onset of data transmission. Prior art methods of overcoming the latch-up problem involve additional start-up circuitry and/or deviations in the QFB structure, and, as a result, require supplementary circuitry and exhibit inferior circuit performance.
Further, an important criteria in designing a QFB DC restorer is the delay which occurs in the feedback loop. Since any delay in the feedback loop of the QFB restorer adversely affects the construction of the signal spectrum at the input of the slicer of the restorer, delay should be kept at a minimum level. In particular, at high data rates, elegant and efficient circuit implementation techniques are critical for keeping the QFB circuit as simple as possible.